For many years, electrostatic coating or spraying has been a widely accepted technique for large scale application of paint, as for example in a production painting line. Typically, spraying involves the movement of very small droplets of "liquid" paint or particles of "powdered" paint from a nozzle to the surface of a part to be coated. The droplet or particle size may vary from less than 0,001 inches (0.025 mm) to greater than 0.1 inches (2.54 mm) depending on the paint viscosity and the air pressure. When the droplets fall on the surface of the part, they flow together to form a continuous wet coat.
Most industrial operations use conventional air spray systems in which compressed air is supplied to a spray gun and to a paint container. At the gun, the compressed air mixes, somewhat violently, with the paint, causing it to break up into small droplets, which are propelled toward the surface of the part to be coated. The process of breaking up the paint into droplets is referred to as "atomization."
Electrostatic spraying involves the movement of electrically charged paint droplets along lines of force that exist between an electrically charged spray gun and a grounded part. Because the paint follows electrical lines of force, it has less of a tendency to miss the part, thus reducing overspray. The paint generally "wraps around" to the rear surface of the part.
Atomization of the paint may be achieved by well-known air, airless or rotational techniques. Air and airless electrostatic guns generally utilize an external ionizing electrode to ionize or charge the air surrounding the part to be coated. Alternatively, electrostatic guns also utilize a metal electrode located within the interior or at the paint container to charge the liquid paint before it is atomized. Rotational methods utilize a rotating electrically charged disk, bell or cone to break up the paint. Atomization is achieved by a combination of centrifugal and electrostatic force.
There are many advantages to electrically coating parts in a production line. For example, electrostatically applied paint is often uniform in thickness, for the simple reason that the charged paint seeks the thinnest part of the wet film so it can better render its charge and adhere to the surface of the part. Smooth application on the edges of a part results from the wrap-around effect of the electrical field. In addition, electrostatic coating of parts in a production paint line facilitates efficiency.
In spite of the many production advantages, there exist safety concerns such as possibilities for electrical shock and fire hazards. A shock may be experienced if a person contacts any part of the coating system not protected by a current limiting circuit. The fire hazard is generally more severe. Sparks generally occur during the electrostatic coating operation. During electrostatic coating it is common to operate a device for dispersing and charging the coating material at a potential of 40,000 volts or higher. In instances, where the coating material is a paint having a volatile solvent, the danger of an explosion or fire from sparking is in fact quite serious.
Fires are also a possibility if electrical arcs occur between charged objects and a grounded conductor in the vicinity of flammable vapors. In any ungrounded metal object which acquires a charge because of its proximity to the gun, if the charge builds to a high enough level, an arc may appear. The usual way to prevent the discharge-arc-fire problem is to ground every object in the paint booth.
For example, in a conventional coating system, parts to be coated are generally transported through a coating zone by a mechanical conveyor. The conveyor is operated at ground potential and the parts are supported on the conveyor by hooks or supports of conductive material to also maintain the parts at ground potential. The coating device includes an electrically charged electrode, preferably at a negative potential with respect to ground. A bad contact between the metallic part to be coated and the conveyor, which can result form a conveyor hook being partly covered with paint or powder during previous passages through the coating zone, can exist. In general, conveyor hooks are not sufficiently cleaned after passing through the coating zone, so that after a number of successive passages, they are covered with a crust which is sufficiently insulating to prevent electric contact between the metallic part to be coated and the grounded conveyor. In consequence, the part to be coated, as it passes close to the electrodes under high tension, acquires electric charges which accumulate on the part until it develops a potential sufficiently high to cause the breakdown of the insulating crust between the part and the hook. This poses a two-fold problem. First, the breakdown is accompanied by a spark capable of developing sufficient energy to ignite the surrounding mixture of air and finely divided coating product. In addition, many of the benefits associated with electrostatic coating are lost if the articles are not properly grounded to terminate the electrostatic field.
To maintain the efficiency of the electrostatic coating process, the spray head and the supporting surfaces must be frequently cleaned. If these surfaces become coated with a layer of dirt or grime, the electrical charge may eventually leak through this layer and be grounded. This typically reduces the charging efficiency of the spray gun as well as decreases attraction to the spray gun. Moreover, the wrap-around effect is reduced and overspray is increased. Frequent cleaning, although necessary, is undesirable in view of the manual labor it involves, which hinders the automated process.
U.S. Pat. No. Re. 28,394 to Point proposes an apparatus for testing electrical contact between metallic objects. However, the apparatus proposed in that patent merely detects sparks with the intention of preventing fires. In addition, U.S. Pat. No. 3,787,707 to Gregg also proposes a spark detector apparatus and method to distinguish disruptive charges in the electrostatic system from a corona discharge or random sparking of other electrical equipment by detecting the presence of repetitive charges.
Thus, a system and method for detecting and controlling ungrounded parts in a production line is desirable.